The studies comprising this proposal are directed at establishing the nature and structural organization of the nuclear RNA precursors to immunoglobulin light chain mRNA and at determining the molecular events in the post-transcriptional processing of these nuclear RNA in the biogenesis of immunoglobulin light chain mRNA. We have developed a general method for cloning complementary DNA (cDNA) and have constructed clones from purified MOPC 21 immunoglobulin kappa light chain mRNA. We have now established the following detailed pathway for the biogenesis of kappa light chain mRNA: 10kb yields 7.8, 5.6, 4.8kb yields 1.2kb transport to cytoplasm yields 1.2kb primary transport; processing intermediates, kappa mRNA. Continuing studies are directed at characterizing the order and mechanisms of splicing events in this pathway. These studies represent an innovative approach for the analysis of gene expression in eukaryotic cells using the powerful resolution afforded by recombinant DNA cloning.